The pressure of a petroleum reservoir, particularly a gas reservoir, decline rather rapidly during production. In order to maintain and prolong production from subsea reservoirs, often involving long transport through a pipeline of the produced fluid, pressure boosting is required.
In FIG. 1 is illustrated the process of a subsea compression station. The rotating equipment is compressors and pumps. The rotational speed of pumps is typically in the range of 3000-4000 rpm while compressors operate typically in the range of 5000-12000 rpm.
Reference is made to Table 1 for understanding of this figure. To give an idea of dimensions, the diameter of the separator in FIG. 1 can be in range of 3 m and height 10 m.
TABLE 1Item #ExplanationaSeparatorbCompressorb′Compressor motorcPumpc′Pump motordLower liquid leveleHigh liquid levelfHigh-high liquid levelgPolishing equipment, e.g. cyclonesg′Lower edge of cycloneshDowncomeriOutlet from downcomerjAntisurge valve with actuatorkAntisurge coolerlCable for supply of electric power to compressor motormCable for supply of electric power to pump motornLiquid recirculation pipeoGas recirculation pipep, p′, p″, p′″ValvesqElectric connector for compressor motorq′Electric connector for compressor motorrLiquid recirculation valve
A typical power requirement for pressure boosting of such a compressor train is 5-15 MW. This, combined with high transmission frequency, limits the length of an electric subsea step out cable, laid out from and controlled from surface (topside or onshore) via a surface variable speed drive (VSD). More specifically, the Ferranti effect, and possibly also other effects, limits the subsea length of high power high frequency electric step out cables to about 40-50 km.
The state of the art of subsea compressors (motor-compressors) are indicated in FIG. 2 where the main components are the compressor which is driven by an electric high speed motor that rotates at the speed that the compressor needs, i.e. the motor rotates at a speed typically in the range of 5000 to 12000 rpm. The motor speed is transmitted to the compressor by at least one shaft that connects motor and compressor. The frequency of the electric power to give this speed for the motor and thereby the compressor must be in the range of approximately 80 to 200 Hz for a 2-pole motor. The shaft power of the compressor motor can typically be in the range of 5-15 MW and possibly larger in the future. Stable transmission of electric power at the high frequencies that the motor requires is feasible if the distance from the power supply, normally from onshore or topsides (surface) is limited to the range of 40-50 km. If the step out distance is more than this, the power transmission through the cable becomes instable and inoperable. In such cases there will be contradictory requirements between the high frequency that the motor needs to give the right speed and the low frequency, say typically 40-70 Hz, which is necessary to have a stable power transmission. This contradiction can be resolved by low frequency power transmission and local increase of frequency by placing a subsea variable speed drive (SVSD) close to the motor.
The atmosphere of the motor-compressor in FIG. 1 will be gas, either the gas being boosted or an inert gas supplied from a reservoir. The term inert gas in the context of this patent description means any gas that is not harmful to the internal materials of the motor, and also of the gear in cases where such a gear is located in the same compartment as the motor. Typically the inert gas can be dry nitrogen or dry methane, however, dry nitrogen is preferable and shall in the context here cover all types of applicable inert gases.
In cases where pumps have liquid filled motor, the motor is filled with an inert liquid, i.e. a liquid that is not harmful to the internal materials of the motor and of the gear in cases where a gear is located in the same compartment as the pump.
It shall be mentioned that only main components necessary for understanding of the state of the art of subsea motor-compressor included in FIG. 2. and the following FIGS. 3-6.
Other vital components necessary for design of a complete operable subsea compressor or pressure booster not included are: Motor gas cooling system, HV power connectors for transmission of power to the motor, LV cables for signal and control of the magnetic bearings, balance piston and others.
However, while subsea processing equipment has gained acceptance over the resent years for being a realistic option, there is more reluctance against electric and electronic equipment, i.e. a perception of that this type of equipment will have low reliability and robustness. This is particularly valid for static subsea variable speed drives, VSD for electric motors. [VSD is also called variable frequency drive (VFD) and frequency converters.] It is therefore a common view in the professional environment that the risk for lost production by application of subsea VSDs is considered to be high and they should if possible be avoided. A SVSD (subsea VSD) will also be large in dimensions and weight and therefore not easy to install and retrieve. The cost will also be high.
A subsea VSD located near the turbomachine will allow a low frequency high power electric power transmission through the subsea step out cable, which allows a far longer step out length. However, the cost of a feasible subsea VSD for a motor of 10 MW can indicatively be 100 MNOK, the weight about 100 tons, the height about 11 m and the diameter about 3 m. But a worse problem is the risk of limited reliability of a subsea VSD.
Even though the subsea VSD contains top quality components, each of very high quality and reliability, the large number of components and the complexity of the structure result in a total subsea VSD reliability that may be a significant problem.
A demand for further improvement still exists, for pressure boosters in general and subsea pressure boosters in particular, and the objective of the present invention is to meet said demand.